The present invention relates to a device and to a process allowing separation or filtration of a continuous phase containing a disperse phase.
Separation processes utilizing filtration techniques are widespread in the food, pharmaceutic, biomedical and water conditioning industries. Such a separation takes account of the size of the disperse phase particles and allows a high-level separation of the phases. One significant field of application relates to the separation of two liquid phases in emulsion.
However, the development of this type of process is checked and its extension to other industry types, such as the petroleum industry, is limited because of maintenance and processing capacity problems. In fact, at the time of the separation, two phenomena reduce the flux flowing through the separation membrane : the concentration polarization, which is expressed by a reversible formation of a disperse phase layer in the vicinity of the membrane surface, and the clogging of the membrane, which may be partly irreversible and results from an obstruction of the pores. These two phenomena cause a clogging of the membrane, which leads to a decrease in the flux flowing through the membrane and therefore to a decrease in the processing capacity of the separation units.
In the description hereafter, what is understood to be the permeate is the fraction of the mixture flowing through a filtering wall and the terms "retentate" or "concentrate" correspond to the fraction which is stopped by the filtering wall. Similarly, what is understood to be a "filtering wall" or "filtration wall" is a porous wall allowing selective filtration and a "solid wall" is a wall made of a homogenous material impervious to the fluids considered.
What is understood to be a "membrane" is a filtering wall comprised, at the surface, of a microporous selective layer and which may be arranged on a macroporous support and/or a grid providing mechanical resistance.
The prior art describes several types of devices designed for limiting the clogging of separation membranes.
Thus, one of the first efforts to improve the existing devices focuses on the structure of the membranes whose pore size is markedly smaller than the size of the particles or of the droplets to be retained in the concentrate and which, if the disperse phase is liquid, are selected so as not to wet the disperse phase. Other improvements proposed consist in achieving separation devices including periodic back pressure and pinpoint chemical washing systems allowing the particles amalgamated on the membrane surface to be drawn away.
Another way of reducing the disperse phase accumulation next to the wall consists in increasing the shear or the turbulence on the membrane surface, for example by carrying out a tangential filtration instead of a frontal filtration. The tangential shear may also be increased by raising the circulation rate of the mixture; this procedure is however disadvantageous in that it reduces the mean residence time of the mixture in the device and consequently the permeate flow rate. This drawback is remedied by recirculating part of the concentrate, which leads to a cost increase.
U.S. Pat. Nos. 1,604,241 and 521,902 describe tangential filtration devices for which the shear on the wall is increased by a helical flow of the mixture passing through several channels. Using several parallel channels is nevertheless disadvantageous in that it decreases the linear circulation rate of the mixture, for the same rate of flow, which leads to a decrease in the shear on the wall and to an increase in the disperse phase accumulation next to the wall.
In patent J-53,102,282, the action of the centrifugal force on a powder admixed with the concentrate is used to clean mechanically and continuously the membrane surface. This process requires an additional stage for separating the powder from the retentate.
U.S. Pat. Nos. 1,800,074 and 2,240,121 relate to dynamic filtration devices in which the membrane surface is brought into rotation or vibration so as to detach the particles caked on the membrane. The drawback of these devices is to implement mechanical parts in motion while having a limited efficiency.